A recent and famous image of HL Tau in deep space marks the first time we've seen a forming planetary system, according to a team of astrophysicists who found that circular gaps in a disk of dust and gas swirling around the young star HL Tau are in fact made by forming planets.

The image of HL Tau, taken in October 2014 by the state-of-the-art Atacama Large Millimeter/submillimeter Array (ALMA) located in Chile's Atacama Desert, sparked a flurry of scientific debate.

The HL Tau system is less than a million years old, about 17.9 billion kilometres in radius and resides 450 light years from Earth in the constellation Taurus.

HL Tau. Credit: Atacama Large Millimeter/submillimeter Array (ALMA)

Those who observed the original image claimed that planets were most likely responsible for carving the gaps, but others remained skeptical. It had been suggested that the gaps, especially the outer three, could not represent forming planets because they are so close together. It was argued that planets massive enough to carve such gaps should be scattered violently by the force of gravity and ejected from the system early on in its development. 

A new study is the first to suggest the gaps are evidence of planetary formation because the gaps are separated by amounts consistent with what's called a special resonant configuration. In other words, these planets avoid violent collisions with each other by having specific orbital periods where they miss each other, similar to how Pluto has avoided Neptune for billions of years despite the two orbits crossing one another. 

Since young systems like HL Tau are shrouded by a thick cloud of gas and dust, they can't be observed using visible light. ALMA resolves that issue by using a series -- or an array -- of telescopes located 15 kilometers apart that use much longer wavelengths. 

While the HL Tau system remains stable in its relatively young age, over billions of years it will act as a "ticking time bomb." Eventually the planets will scatter, ejecting some and leaving the remaining bodies on elliptical orbits like the ones found around older stars. 

The findings will be published in the upcoming edition of Astrophysical Journal.